add explicit matrix conversion operators to quat

[plasmacel]

as you stated in issue #274, adding explicit constructors to matrix types would break the spirit of the library. however this feature can be achieved by adding explicit conversion operators to glm::quat. this way the core profile (the matrix types) remains unpolluted, according to the GLSL standard. so the feature only available if GTC_quaternion extension is used. it is a more C++11 friendly way of type conversion and also preserves the concept of the library.

[richardeakin]

IMO, I don't think overloading operator() is very explicit at all and can make code refactoring a headache. I personally prefer the mat3_cast() if that's what you need. Or, something like a to_mat3() member function (though possibly this isn't really in functional spirit of the library).

[plasmacel]

you overload "operator tmat3x3<T,P>" and "operator tmat4x4<T,P>", not at all "operator()".

in C++11 you can define explicit cast operators: http://en.cppreference.com/w/cpp/language/cast_operator

this way you can cast a quat to mat3 or mat4 with a standard static_cast

[richardeakin]

Ah I see, sorry my mistake as this is very much explicit. :) Personally I'd use the conversion function as it is less typing, but I can see how others might prefer to use this technique.

[plasmacel]

yeah :), however you can use it with c-style casts too, they are also less typing:

glm::quat q(1.0f, 0.0f, 0.0f, 0.0f);
glm::mat3 m = glm::mat3(q);

[Groovounet]

Very nice. I'll pull to master branch.

Thanks for contributing,
Christophe